Computer implemented method of determining a base curve for a spectacle lens and method of manufacturing a spectacle lens

ABSTRACT

A computer implemented method of determining a base curve value representing a base curve for a front surface of a spectacle lens comprises the steps of receiving individual prescription data and determining the base curve value for the front surface of the spectacle lens based on the prescription data. The base curve value is calculated from the received prescription data based on a functional relationship between one or more values included in the prescription data and the base curve value.

FIELD OF THE INVENTION

The present invention relates to a computer implemented method of determining a base curve value representing a base curve for a spectacle lens, that is, an ophthalmic lens which is worn in front of the eyeball without being in contact with it. In addition, the invention relates to a method of manufacturing a spectacle lens.

BACKGROUND OF THE INVENTION

A free-form surface of a spectacle lens is a surface which may freely be formed during the manufacturing process and which does not need to show axial symmetry or rotational symmetry. In particular, a free-form surface may lead to different dioptric powers (spherical and/or astigmatic) in different sections of the surface. The use of free-form surfaces allows for improving the quality of spectacle lenses with regard to imaging quality experienced by the wearer. For example, by use of a free-form surface a spectacle lens may be optimized in view of the individual prescription data as well as the individual as worn-position of the wearer for whom it is manufactured. The possibility of providing a free-form surface exists for single-vision lenses as well as for multifocal or progressive-power lenses where the term “single-vision lens” refers to a spectacle lens designated to provide a single dioptric power and the term “progressive-power lens” refers to a spectacle lens with at least one progressive surface that provides increasing power as the wearer looks down. A progressive-power lens includes a near portion and a distance portion where the terms “near portion” and “distance portion” refer to that portion of the multifocal or progressive-power lens having the dioptric power for near vision and that portion of a multifocal or progressive-power lens having the dioptric power for distance vision, respectively. The difference between the near power and the distance power experienced by the wearer is called addition power. Between the near portion and the distance portion there is an intermediate corridor in which the power experienced by the wearer decreases from the dioptric power for near vision to the dioptric power for distance vision and in which the vision for the wearer is clear. Free-form surfaces of progressive power lenses include a larger number of parameters, which may be taken into account in the calculation of the surface than in the calculation of the free-form surfaces for single vision lenses, for example, the length of the intermediate corridor, the position of the intermediate corridor or the addition power.

A free-form surface may be located at the front surface or the rear surface of a spectacle lens. However, it is also possible that both the front surface and the rear surface are free form-surfaces. In all three cases there usually exists a set of base curves represented by a base curve value. The base curve value is given by the nominal surface power typically of the front surface. In this context, the surface power is the difference between the refractive index in front and behind the surface multiplied by the curvature of the surface. If the curvature of the surface is not constant, as it is for example the case with a free-form surface, the curvature by which the difference between the refractive index in front and behind the surface is to be multiplied resembles a mean curvature at a reference point of the surface or an average curvature that is averaged over the whole surface. Often, the base curves are given for a nominal refractive index of 1.53, even if the material on which the spectacle lens is made of has another refractive index. In this case, the base curve value needs to be transformed for the actual refractive index for optical calculations. The base curve that is to be used for given prescription data depends at least on the spherical power and/or the cylindrical power of the spectacle lens. If applicable, the base curve to be used may depend on further parameters such as addition power, length of the intermediate corridor, et cetera. Typically, there exist so-called base curve charts which associate a number of base curves values to a number of combinations of spherical power and cylindrical power.

In case of a free-form surface on the rear surface of the spectacle lens the front surface is preferably given by a spherical surface as defined by the base curve value and the shape of the free-form surface is configured such that the dioptric power according to the prescription data is provided when the wearer of the spectacle lens is looking through the spectacle lens.

In case of a free-form surface on the front surface of the spectacle lens the focal power resulting from the prescription data is preferably realized through a suitable spherical or toric shape of the rear surface of the spectacle lens.

A main reason for providing a number of base curves is to provide a meniscus-like shape of the spectacle lenses, that is, with a convex front surface and a concave rear surface. The concave rear surface of the spectacle lens shall have preferably a surface power between at least zero and at least 10 diopter for the whole rear surface.

Usually the number of base curves is limited in order to minimize the number of molds for forming semi-finished lens blanks and stocking costs. Therefore, typically the number of base curves is below 10.

Therefore, each base curve is typically used for a domain of values of focal powers, where each domain includes intervals of the values of the different powers, for example, an interval of spherical power and an interval of cylindrical power. At the boarders of the domains the base curve values representing the bending of the front surface show steps which are typically in the order of one diopter as referred to the refractive index of 1.53. However, the steps may as well be larger than one diopter, for example 1.25 or 1.5 diopter.

There is a desire, to choose the most suitable base curve for given prescription data and as-worn data. A method of choosing a suitable base curve out of a set of base curves is described in U.S. Pat. No. 8,313,194 B2. In this method, a customized base curve is selected from the list of base curves wherein the customized base curve is selected according to a customization criterion. However, with advancing individualization of spectacle lenses finding suitable base curves is still an issue.

SUMMARY OF THE INVENTION

An aspect of the invention refers to a computer implemented method of determining a base curve value. In the description, the following terms are used as defined below.

“Spherical power” is a power of a spectacle lens that brings a paraxial pencil of parallel light to a single focus, where a paraxial pencil of parallel light is a pencil of light in which the distance of the light rays contained in the pencil of light from the optical axis is small and the angles of the rays of light with respect to the optical axis can be approximated according to sin α≈α.

“Astigmatic power” of a spectacle lens refers to the ability of a spectacle lens to bringing a paraxial pencil of parallel light to two separate line foci mutually at right angles. In this context, the term “principal meridian” refers to one of two mutually perpendicular meridians of a cylindrical power lens which are parallel to the two lines of foci where the term “meridian” refers to a plane which contains the center of curvature of a surface and the normal-vector at the center of curvature. Related to the astigmatic power is the “cylindrical power” which stands for the difference of powers in the two principal meridians. The direction of the principal meridian which is chosen as reference for the cylindrical power is called “cylinder axis”.

“Prismatic power” refers to the value of the prismatic effect at the configured reference point as defined in DIN EN ISO 13666: 2013-10, section 10. DIN EN ISO 13666: 2013-10 is incorporated herein by reference.

The term “focal power” is used as a generic term for the terms “spherical power” and “astigmatic power” and the term “dioptric power” is used as a generic term for the terms “focal power” and “prismatic power”.

The term “prescription data” or “individual prescription data” is used as generic term for a set of optical characteristics of the spectacle lens like a value for the spherical power, a value for cylindrical power, a direction of the cylinder axis, and, if applicable, a value for the addition power as determined by an ophthalmologist or an optometrist in order to correct the individual vision of the wearer. In addition, the prescription may contain further values like, for example, a value for the prismatic power.

The term “as-worn position” refers to a position and orientation of the spectacle lens relative to the eyes and face during wear (see DIN ISO 13666:2013-10, sections 9.15) and includes at least values for the back vertex distance, the face form angle and the pantoscopic angle. The “face form angle” is the angle between the plane of the spectacle front and the plane of the right lens shape, or of the left lens shape, the term “pantoscopic angle” refers to an angle in the vertical plane between the normal to the first front surface of the spectacle lens at its boxed center, that is, at the intersection of the horizontal and vertical center lines, and the term “back vertex distance” refers to the distance between the apex of the cornea and the rear surface of the spectacle lens in a defined viewing direction (see DIN ISO 13666:2013-10, sections 5.27 and 17).

The term “frame data” includes the geometry of the spectacle frame and the coordinates of the centration point (see DIN ISO 13666:2013-10, section 17).

The functional relationship is a real function as it is defined in mathematics. The functional relationship is preferably a continuous functional relationship.

A continuous functional relationship means a real continuous function as it is defined in mathematics: the function f is continuous at a point h of its domain D, if for every sequence (x_(n)) in D, that tends toward h, the sequence F(x_(n)) always converges to f(h), that is, f(x_(n))->f(h).

According to a first aspect of the present invention a computer implemented method of determining a base curve value representing a base curve for a front surface of a spectacle lens is provided. The method includes a step of receiving prescription data, for example, through manual input by use of a human-machine interface such as a keyboard, a voice recognition unit, a touch screen, et cetera, or through an electronic interface. From the received prescription data the base curve value is calculated for the individual prescription data based on a continuous, non-constant functional relationship between at least one value included in the prescription data on the one side and the base curve value on the other side.

In an embodiment of the computer implemented method of determining a base curve value the prescription data comprises at least a value for spherical power. Alternatively, the computer implemented method of determining a base curve value comprises at least a value for spherical power and a value for cylindrical power or spherical power and object distance, preferably spherical power and variable object distance, or spherical power and cylindrical power and optionally object distance or spherical power and cylindrical power and axis and optionally object distance or spherical power and prismatic power and optionally object distance or spherical power and cylindrical power and prismatic power and optionally object distance, each value included in the individual prescription data for far vision and/or near vision.

In this case calculating the base curve value may include calculating the base curve value based on a functional relationship between the value for spherical power contained in the prescription data on the one side and the base curve value on the other side, or on a functional relationship between values for spherical power and cylindrical power contained in the prescription data on the one side and the base curve value on the other side. In this case possible values for spherical power and cylindrical power which may be included in the prescription data may form at least two domains of values. Then, the functional relationship between values included in the prescription data and the base curve value may depend on the domain the values for spherical power and cylindrical power contained in the individual prescription data are part of.

In a further embodiment of the computer implemented method of determining a base curve value the method may further comprise receiving as-worn position data and/or frame data and taking into account the received as-worn position data and/or the received frame data when calculating the base curve value. Like the individual prescription data the as-worn position data and/or frame data may be received from manual input through human machine interface, from an electronic interface, et cetera.

According to a second aspect of the present invention this objective is obtained by a method of manufacturing a spectacle lens, preferably individually adapted to a wearer, where the method comprises the steps of:

-   -   providing individual prescription data and optionally individual         as-worn position data for the spectacle lens to be manufactured;     -   determining a base curve value for the front surface of the         spectacle lens element based on the individual prescription data         and optionally based on the individual as-worn position data;     -   providing a spectacle lens element with a front surface and a         rear surface; and     -   machining the spectacle lens element based on the individual         prescription data and optionally based on the individual as-worn         position data;         wherein     -   determining the base curve value is done by calculating it from         the received individual prescription data based on a continuous,         non-constant functional relationship between at least one value         included in the individual prescription data and the base curve         value, wherein the at least one value included in the individual         prescription data comprises at least     -   spherical power and object distance or     -   spherical power and cylindrical power and optionally object         distance or     -   spherical power and cylindrical power and axis and optionally         object distance or     -   spherical power and prismatic power and optionally object         distance or     -   spherical power and cylindrical power and prismatic power and         optionally object distance,         each value included in the individual prescription data for far         vision and/or near vision and     -   providing the spectacle lens element includes determining the         front surface and the rear surface of the spectacle lens element         so as to obtain the spectacle lens with a base curve with the         determined base curve value on the front surface.

Individually adapted to a wearer means that the optical correction of the vision of the wearer is performed by using the individual prescription data and optionally the data of the as-worn position.

According to an aspect of the present invention the machining of the spectacle lens element includes machining of the front surface and/or the rear surface of the spectacle lens element so as to obtain the spectacle lens with a base curve with the determined base curve value on the front surface.

According to an embodiment, determining the base curve value is done by using an inventive computer implemented method of determining a base curve value.

In particular, the rear surface of the lens element is calculated and machined such that the spectacle lens preferably in its as-worn position has a dioptric power according to the prescription data.

The method of manufacturing a spectacle lens individually adapted to a wearer may, in particular, be applied in manufacturing spectacle lenses which are progressive addition lenses. In this case it may be advantageous to form a progressive surface of the progressive addition lens on the front surface of the spectacle lens element.

In an embodiment of the method of manufacturing a spectacle lens individually adapted to a wearer frame data containing data about the geometry of the spectacle frame is provided and machining the spectacle lens element is also based on the frame data.

According to a third aspect of the present invention, a computer program is provided with program code for performing the method steps according to the computer implemented method of determining a base curve value representing a base curve for a front surface of a spectacle lens when the computer program is loaded or executed in a computer.

The invention provides methods of determining continuous base curve values as a function of parameters in the prescription data and to manufacture the respective base curves by machining the front surface and/or the rear surface of a spectacle lens element like a spectacle lens blank or a semi-finished spectacle lens blank. This allows for avoiding steps between the base curve values related to values contained in the prescription data which differ only slightly from each other. Using a base curve chart as it is done in the state of the art may lead to the situation that values, for example, for spherical power, contained in the prescription data which differ only slightly from each other lead to different base curve values which differ by one diopter or more at a refractive index of 1.53.

In an embodiment of the invention the minimum curvature of the rear surface of a spectacle lens element is constant for a domain of prescription data, for example, a set of combinations of spherical and cylindrical data, preferably for prescription data with sphere ≥0 and the base curve results from the dioptric requests of the individual prescription data and optionally the data of the as-worn position and optionally the frame data resulting in thin, flat and aesthetic spectacle lenses for the whole domain of individual prescription data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings wherein:

FIG. 1 shows a flowchart showing how a spectacle lens individually adapted to a wearer is manufactured;

FIG. 2 shows a flowchart showing how a base curve for a spectacle lens is determined;

FIG. 3 shows a base-curve selection chart according to the state of the art; and,

FIG. 4 shows a chart with base curves values calculated according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A detailed description of an embodiment of an inventive method of manufacturing a spectacle lens individually adapted to a wearer will be described with reference to the flowchart shown in FIG. 1.

In the method, individual prescription data of the wearer as well as individual as-worn position data of the wearer are provided in steps M1 and M1′. In addition, frame data are also provided in step M1″. In the present embodiment, the prescription data contains values of spherical power and cylindrical power together with an indication of the direction of the cylinder axis, where the values of spherical power and cylindrical power may also include zero so that the spectacle lens resulting from an inventive method may have a spherical power of 0 diopter or an cylindrical power of 0 diopter. However, in the general case the prescription data contains a non-zero value for spherical power and a non-zero value for cylindrical power. In addition to the values for spherical power and cylindrical power the prescription data may contain additional values, i.a. a value for representing addition power and/or a value representing prismatic power. The as-worn position data contains in the present embodiment a value for the back vertex distance, a value for the pantoscopic angle and a value for the face form angle and the frame data contains data relating to the geometry of the spectacle frame.

In a next step M2 a base curve value representing a base curve for the spectacle lens to be manufactured is determined. The base curve value is a measure for the nominal surface power to be given to the front surface of the spectacle lens. According to the embodiment, the base curve value—and thus the base curve, is determined based on the combination of values given in the prescription data and, if applicable, in the as-worn position data and/or in the frame data. How the base curve value is determined will be described later with respect to the flowchart of FIG. 2. The nominal surface power of the front surface does not need to be the exact final surface power of the front surface if the free-form surface is formed on the front surface.

Next, in step M3 the shapes of the front and rear surfaces of the spectacle lens are determined. In the present embodiment, a free-form surface is to be formed on the front surface. In this case, a suitable spherical or toric rear surface is determined such that the free-form surface on the front surface has an average curvature, that is, a curvature averaged over the whole surface, or a mean curvature at a reference point of the front surface, which leads to a surface power that approximately matches the base curve value. Determining the spherical or toric rear surface typically is done iteratively by means of ray tracing. In the ray tracing process, a starting geometry of the spectacle lens is given. The starting geometry may include as front surface a known free-form front surface or a spherical front surface, which has the required base curve value, and as rear surface a given spherical or toric surface. Then, the spherical or toric rear surface is varied until the calculated power of the spectacle lens coincides with the required power for correcting the ametropia of the wearer, preferably with the spectacle lens being in the as-worn position.

With the so determined spherical or toric rear surface the free-form surface is optimized using ray-tracing. During this optimization, the average curvature or mean curvature, respectively, does not change significantly anymore, so that the front surface keeps the required base curve value.

Next, in step M4 a lens element is provided which has a front surface and a rear surface. The lens element may be a spectacle lens blank or a semi-finished spectacle lens blank. In a semi-finished spectacle lens blank the front surface usually has one of a number of surface powers which may be chosen such that the surface power of the front surface corresponds to the determined base curve value as closely as possible. However, it is not mandatory to provide a semi-finished spectacle lens blank with a surface power of its front surface which corresponds to the base curve value. In particular, instead of a semi-finished spectacle lens blank a spectacle lens blank with flat front and rear surfaces, that is, a cylindrical spectacle lens blank, may be used as well. The only restriction is, that the spectacle lens blank needs to be thick enough to allow manufacturing the spectacle lens with the determined base curve.

Once the base curve value is determined, the shapes of the front and rear surfaces of the spectacle lens are determined and the spectacle lens element is provided, the spectacle lens element is machined in step M5 based on the prescription data and the as-worn position data so as to form a front surface and a rear surface having the determined shapes, such that a spectacle lens is formed that is individually adapted to the wearer. Machining the spectacle lens element includes for example machining the front surface so as to form the base curve represented by the determined base curve value on the front surface. In case the spectacle lens to be manufactured is a single vision lens or a progressive power lens with the free-form surface formed on the rear surface the rear surface will be machined according to the prescription data so as to form a rear surface which together with the front surface allows the spectacle lens to fulfill the individual optical needs given in the prescription data.

If, on the other hand, the spectacle lens is a progressive addition lens with the free-form surface formed on the front surface the front surface is machined according to numerical data describing the free-form surface. This numerical data is based on the individualized power to be achieved and optionally on the as-worn position. It is also possible to form in addition to the free-form surface on the front surface a further free-form surface on the rear surface. Then both free-form surfaces together provide for the addition power of the progressive addition lens.

The machining performed in step M4 may include milling and polishing the front surface and/or the rear surface under computer numeric control for producing the free-form surface and fine turning on which follows a polishing step. After the lens has been machined one or more coatings may be applied on one or more of the spectacle lens surfaces.

FIG. 2 shows the method of determining a base curve value for the spectacle lens. In the present embodiment, the method is implemented on a computer and comprises the step of receiving prescription data (step D1) through a computer interface. In addition, an optional step of receiving as-worn position data (step D1′) through the computer interface and/or an optional step of receiving frame data (step D1″) with data relating to the geometry of the spectacle frame through the computer interface may be present.

Next, in step D21 the program evaluates the values contained in the prescription data to see whether these values belong to one of a number of domains the values in the prescription data may be in. In the present embodiment, the base curve value is determined based on the values of spherical power and cylindrical power given in the prescription data. The spherical power may assume values between −8 diopter and +7 diopter and the cylindrical power may assume values between 0 and +4 diopter. A first domain of values of the prescription data contains, in the present embodiment, all combinations of values for spherical power and cylindrical power in which the spherical power is below −7 diopter. If it is determined in step D21 that the value for spherical power given in the prescription data is below −7 the method proceeds to step D31 in which a functional relationship between the spherical power on the one side and the base curve value on the other side is applied to determine the base curve value from the value of the spherical power. The functional relationship used in step D31 is valid for all values of the prescription data which are in the first domain.

If, on the other hand, it is determined in step D21 that the value for the spherical power is not below −7 the method proceeds to step D22 in which it is determined whether the combination of the value for spherical power and the value for cylindrical power given in the prescription data is in the second domain. The second domain contains all combinations of values for spherical power and cylindrical power in which the value for spherical power is between −7 and −4.75. In case of Yes, the method proceeds to step D32 in which a second functional relationship is applied which relates the value of the spherical power to the base curve value. In case the spherical power is not in the interval between −7 diopter and −4.75 diopter the method proceeds to a further step in which it is determined whether the values given in the prescription data lie in a third domain. This proceeds until all n domains have been checked and the values given in the prescription data have been associated to one of the domains. In other words, the method determines to which domain the values the combination of spherical power and cylindrical power given in the prescription data belongs and applies the corresponding functional relationships between the value spherical power and the base curve value or between the spherical power and the cylindrical power and the base curve value. At the end, the determined base curve value is output in step D4.

An example for a program code by which the domain the combination of spherical power and cylindrical power given in the prescription data belongs to and the base curve value can be determined based on the values of spherical power and cylindrical power given in the prescription data is, for example

  if (fSph < −7.00) then   fGK = 1.00+(8.00+fSph)*0.40/1.00  elseif (fSph < −4.75) then   fGK = 1.40+(7.00+fSph)*0.90/2.25  elseif (fSph < −3.00) then   fGK = 2.30+(4.75+fSph)*0.90/1.75  elseif ((fSph < −1.50).and.(fSph+fZyl < 1.50)) then   fGK = 3.20+(3.00+fSph)*0.80/1.50   if (fSph+fZyl > 1.00) fGK = fGK+.30*(fZyl−3.00)  elseif (fSph+fZyl < 1.50) then   fGK = 4.00  elseif ((fSph+fZyl < 4.50)) then   fGK = 4.00+(fSph+fZyl−1.50)*2.50/3.00  else   fGK = 6.50+(fSph+fZyl−4.50)*1.00/2.50  endif where fGK stands for the base curve value in diopter, fSph stands for the value of the spherical power in diopter of the prescription data and fZyl stands for the cylindrical power in diopter of the prescription data.

FIG. 3 shows a base curve chart according to the state of the art showing base curve values for combinations of spherical power and cylindrical power with the spherical power being in the range of −8 to +7 diopter and the cylindrical power being in the range of 0 to 4 diopter. FIG. 4 shows for the same combinations of values for spherical power and cylindrical power the base curve values determined by use of the above program code. The base curve values provided in FIGS. 3 and 4 are based on the organic material with a refractive index of 1.50, for example, poly(allyl diglycol carbonate) also known as CR39.

As can be seen from FIG. 3, in the state of the art base curve chart there are steps between neighboring values of spherical power and/or cylindrical power which are one diopter or more at a refractive index of 1.53. For example, in the base curve chart shown in FIG. 3 the base curve value for a spherical power of 1.25 diopter and a cylindrical power of 0.50 diopter would be 5.25 diopter whereas the base curve value for a spherical power of 1.00 diopter and a cylindrical power of 0.50 diopter would be 4.00 diopter. This means a step of 1.25 diopter is present between the spherical power of 1.00 diopter and 1.25 diopter. If, for example, the left and right lenses of a spectacle have spherical powers of 1.00 diopter and 1.25 diopter, respectively, and both have a cylindrical power of 0.50 diopter the base curve chart of the state of the art would lead to a situation where the left spectacle lens would have a base curve value of 4.00 diopter and the right spectacle lens would have a base curve value of 5.25 diopter. In other words, the left and right spectacle lenses would look rather different although the difference in spherical power is rather small. In the chart showing the base curve values as calculated by the above program code the differences in the base curve values of neighboring values of spherical power and/or neighboring values of cylindrical power are always small. For example, for the prescription data with a value of 1.25 diopter for the spherical power and a value of 0.5 diopter for the cylindrical power, a method according to the invention provides a base curve value of 4.21 diopter, and for a value of 1.00 diopter for the spherical power and a value of 0.50 diopter for the cylindrical power the inventive method provides a base curve value of 4.00 diopter (see FIG. 4). Hence the difference is only 0.21 diopter as compared to 1.25 diopter in the state of the art base curve chart. Such a small difference is barely visible in the finished spectacle lenses. Hence, the invention allows for producing more aesthetic spectacles.

In general, aesthetic reasons lead to the desire to have the base curve for higher positive spherical powers as flat as possible. With the inventive method flatter base curves can be achieved. For example, assume a spherical power of 4.00 diopter with a cylindrical power of 0.75 diopter. According to the state of the art base curve chart shown in FIG. 3 this would lead to a base curve of 8.00 diopter. According to an inventive method a base curve of 6.60 would be sufficient. Furthermore, sometimes spectacle glasses with a desired base curve are ordered. If, for example a spectacle lens is ordered with a spherical power of 3.25 diopter and a cylindrical power of 0 diopter with a desired base curve of 5.5 diopter the state of the art base curve chart of FIG. 3 would lead to a base curve of 6.5 diopter since the base curve of 5.25 diopter would already be to flat for producing the spectacle lens. With the inventive method, a base curve of 5.46 diopter would be provided which matches the desired base curve very closely.

With the inventive method a base curve chart with not normalized values (for example not normalized to a step size of 0.25 diopter) may also be generated.

In the chart of FIG. 4 the spherical power and the cylindrical power are given in steps of 0.25 diopter for comparing it with the base curve chart of FIG. 3. However, the values of spherical power and/or cylindrical power may be given in a continuous fashion or in much smaller steps than shown in the chart of FIG. 4. Of course determining the base curve value may also be done in the same way for values of spherical power and/or cylindrical power that are given in steps that are smaller than 0.25 diopter, for example for values that are given in steps of 0.01 diopter, or even less.

Furthermore, the method offers the possibility to adapt the base curve on the front surface in an optimal fashion to the prescription data, the as-worn position and the data of the spectacle frame. In particular, orders for positive spherical power and positive cylindrical power often contain very flat base curves for aesthetic reasons. The requirements of the bending of the rear surface (for example given through a minimum value) may include the absolute surface power of the rear surface over the whole surface or almost the whole rear surface or a minimum value for the average curvature of the rear surface, that leads to a minimum bending, that is, a minimum base curve value of the front surface. Starting from the requirements of the bending of the rear surface can be determined such that the finished spectacle lens realizes the desired spherical and cylindrical power, the required bending of the rear surface and optionally the as-worn position. In this context the data of the spectacle frame influences a glass thickness of the spectacle lenses and thereby also the curvature required for the front surface and the rear surface. Hence, the invention allows for taking the frame data into account. The base curves values can then not be represented by a chart as shown in FIG. 4 because the base curve values also depend on the individual as-worn position and the data of the spectacle frame. However, in any case a suitable base curve can be calculated through a functional relationship taking into account not only spherical power and/or cylindrical power but also values of the as-worn position and of the frame data. In the end the wearer receives an aesthetically optimized spectacle glass which only leads to a small magnification or small diminution of the visual perception of the eyes behind the spectacle lens.

The present invention has been described by use of specific embodiments of the invention for illustrative reasons only. A person skilled in the art is aware of possible deviations from the embodiments. For example, although eight functional relationships are used in the program code shown with respect to the present embodiment a larger or smaller number of functional relationships could be used where the larger or smaller number of functional relationships comes along with a larger or smaller number of domains for the values given in the prescription data. Moreover, FIGS. 1 and 2 show the prescription data, the as-worn position data and the frame data to be provided/received simultaneously. However, it would also be possible to provide/receive these data serially in any possible order.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A computer implemented method of determining a base curve value representing a base curve for a front surface of a spectacle lens, the method comprising the steps of: receiving individual prescription data; and, determining the base curve value for the front surface of the spectacle lens from the received individual prescription data; wherein said determining the base curve value is done by calculating it from the received individual prescription data based on a continuous, non-constant functional relationship between at least one value included in the individual prescription data and the base curve value, wherein the at least one value included in the individual prescription data comprises at least one of: spherical power and object distance, spherical power and cylindrical power and optionally object distance, spherical power and cylindrical power and axis and optionally object distance, spherical power and prismatic power and optionally object distance, and, spherical power and cylindrical power and prismatic power and optionally object distance; and, wherein each value included in the individual prescription data is for at least one of far vision and near vision.
 2. The computer implemented method of claim 1, wherein the values representing spherical power and representing cylindrical power which are included in the individual prescription data are divided into at least two domains of values and the continuous, non-constant functional relationship between values included in the individual prescription data and the base curve value depends on the domain that the value representing spherical power and the value representing cylindrical power contained in the prescription data are part of.
 3. The computer implemented method of claim 1, wherein said determining the base curve value is done by calculating it from the received individual prescription data based on a continuous, non-constant functional relationship between at least the spherical power and a preset curvature of the rear surface.
 4. The computer implemented method of claim 1, wherein the minimum curvature of the rear surface is constant for a domain of prescription data and the base curve results from the dioptric requests of the individual prescription data and optionally the data of the as-worn position and optionally the frame data.
 5. The computer implemented method of claim 1 further comprising the steps of: receiving at least one of as-worn position data and frame data; and, taking into account at least one of the received as-worn position data and the received frame data when calculating the base curve value.
 6. A method of manufacturing a spectacle lens comprising the steps of: providing individual prescription data and optionally individual as-worn position data for the spectacle lens to be manufactured; determining a base curve value for the front surface of the spectacle lens element based on the individual prescription data and optionally based on the individual as-worn position data; providing a spectacle lens element with a front surface and a rear surface; and machining the spectacle lens element based on the individual prescription data and optionally based on the individual as-worn position data; wherein said determining the base curve value is done by calculating it from the received individual prescription data based on a continuous, non-constant functional relationship between at least one value included in the individual prescription data and the base curve value, wherein the at least one value included in the individual prescription data comprises at least one of spherical power and object distance, spherical power and cylindrical power and optionally object distance, spherical power and cylindrical power and axis and optionally object distance, spherical power and prismatic power and optionally object distance, and spherical power and cylindrical power and prismatic power and optionally object distance, wherein each value included in the individual prescription data is for at least one of far vision and near vision; and, wherein said providing the spectacle lens element includes determining the front surface and the rear surface of the spectacle lens element so as to obtain the spectacle lens with a base curve with the determined base curve value on the front surface.
 7. The method of claim 6, wherein said determining the base curve value is done by using a computer implemented method which calculates the base curve value from the provided individual prescription data based on a continuous, non-constant functional relationship between at least one value included in the individual prescription data and the base curve value.
 8. The method of claim 8, wherein at least one of the front surface and the rear surface of the lens element is machined such that the spectacle lens in its as-worn position has a dioptric power according to the individual prescription data.
 9. The method of claim 6, wherein said spectacle lens is a progressive addition lens.
 10. The method of claim 9, wherein a progressive surface of the progressive addition lens is formed on the front surface of the spectacle lens element.
 11. The method of claim 6, wherein the machining of the front surface includes forming a free-form surface and in which frame data are provided and the machining of the spectacle lens element is also based on the frame data.
 12. A computer program comprising: a program code stored on a non-transitory computer readable medium; said program code being configured to, when the computer program is loaded or executed in a computer, receive individual prescription data and determine a base curve value for a front surface of a spectacle lens from the received individual prescription data; wherein the determining the base curve value is done by calculating it from the received individual prescription data based on a continuous, a non-constant functional relationship between at least one value included in the individual prescription data and the base curve value, wherein the at least one value included in the individual prescription data comprises at least one of: spherical power and object distance, spherical power and cylindrical power and optionally object distance, spherical power and cylindrical power and axis and optionally object distance, spherical power and prismatic power and optionally object distance, and, spherical power and cylindrical power and prismatic power and optionally object distance; and, wherein each value included in the individual prescription data is for at least one of far vision and near vision. 